Interest in the biological control of insect pests has arisen as a result of disadvantages of conventional chemical pesticides. Chemical pesticides generally affect beneficial as well as nonbeneficial species. Insect pests tend to acquire resistance to such chemicals so that new insect pest populations can rapidly develop that are resistant to these pesticides. Furthermore, chemical residues pose environmental hazards and possible health concerns. Biological control presents an alternative means of pest control which can reduce dependence on chemical pesticides.
The primary strategies for biological control include the deployment of naturally-occurring organisms which are pathogenic to insects (entomopathogens) and the development of crops that are more resistant to insect pests. Approaches include the identification and characterization of insect genes or gene products which may serve as suitable targets for insect control agents, the identification and exploitation of previously unused microorganisms (including the modification of naturally-occurring nonpathogenic microorganisms to render them pathogenic to insects), the modification and refinement of currently used entomopathogens, and the development of genetically engineered crops which display greater resistance to insect pests.
Viruses that cause natural epizootic diseases within insect populations are among the entomopathogens which have been developed as biological pesticides. Baculoviruses are a large group of viruses which infect only arthropods (Miller, L. K. (1981) in Genetic Engineering in the Plant Sciences, N. Panopoulous, (ed.), Praeger Publ., New York, pp. 203-224; Carstens, (1980) Trends in Biochemical Science 52:107-110; Harrap and Payne (1979) in Advances in Virus Research, Vol. 25, Lawfer et al. (eds.), Academic Press, New York, pp. 273-355). Many baculoviruses infect insects which are pests of commercially important agricultural and forestry crops. Such baculoviruses are potentially valuable as biological control agents. Four different baculoviruses have been registered for use as insecticides by the U.S. Environmental Protection Agency. Among the advantages of baculoviruses as biological pesticides is their host specificity. Not only do baculoviruses as a group infect only arthropods, but also individual baculovirus strains usually only infect one or a few species of insects. Thus, they pose no risk to man or the environment, and can be used without adversely affecting beneficial insect species.
Baculovirus subgroups include nuclear polyhedrosis viruses (NPV), granulosis viruses (GV), and nonoccluded baculoviruses. In the occluded forms of baculoviruses, the virions (enveloped nucleocapsids) are embedded in a crystalline protein matrix. This structure, referred to as an inclusion or occlusion body, is the form found extraorganismally in nature and is responsible for spreading the infection between organisms. The characteristic feature of the NPV viruses is that many virions are embedded in each occlusion body. The NPV occlusion bodies are relatively large (up to 5 micrometers). Occlusion bodies of the GV viruses are smaller and contain a single virion each. The crystalline protein matrix of the occlusion bodies of both forms is primarily composed of a single 25,000 to 33,000 dalton polypeptide which is known as polyhedrin or granulin. Baculoviruses of the nonoccluded subgroup do not produce a polyhedrin or granulin protein, and do not form occlusion bodies.
In nature, infection is initiated when an insect ingests food contaminated with baculovirus particles, typically in the form of occlusion bodies. The occlusion bodies dissociate under the alkaline conditions of the insect midgut, releasing individual virus particles which then invade epithelial cells lining the gut. Within a host cell, the baculovirus migrates to the nucleus where replication takes place. Initially, certain specific viral proteins are produced within the infected cell via the transcription and translation of so-called "early genes." Among other functions, these proteins are required to allow replication of the viral DNA, which begins 4 to 6 hours after the virus enters the cell. Extensive viral DNA replication proceeds up to about 12 hours post-infection (pi). From about 8 to 10 hours pi, the infected cell produces large amounts of "late vital gene products." These include components of the nucleocapsid which surrounds the viral DNA during the formation of progeny virus particles. Production of the progeny virus particles begins around 12 hours pi. Initially, progeny virus migrate to the cell membrane where they acquire an envelope as they bud out from the surface of the cell. This nonoccluded virus can then infect other cells within the insect. Polyhedrin synthesis begins from 12 to 18 hours after infection and increases to very high levels by 24 hours pi. At that time, there is a decrease in the number of budded virus particles, and progeny virus are then embedded in occlusion bodies. Occlusion body formation continues until the cell dies or lyses. Some baculoviruses infect virtually every tissue in the host insect so that at the end of the infection process, the entire insect is liquified, releasing extremely large numbers of occlusion bodies which can then spread the infection to other insects. (Reviewed in The Biology of Baculoviruses, Vol. I and II, Granados and Federici (eds.), CRC Press, Boca Raton, Fla., 1986.)
One significant disadvantage to using baculoviruses as pesticides is the length of time between virus ingestion and insect death. During this time, the pest insect continues to feed and damage crops. Because a grower is unlikely to apply the pesticide until after an infestation is apparent, it is critical that the time of feeding be minimized.
What is needed is a biological pesticide which reduces feeding by the insect before death. A biological pesticide is preferred because it creates less of an environmental hazard than a chemical pesticide. More rapid insect death is also desirable. The exploitation of (a) gene(s) that encodes (a) protein(s) which controls insect development and its incorporation into various organisms will allow improved biological control of insect pests.